Add abspath parameter for specifying a site root

Relates to #18 and is derived from jongeho1's work in #36 

If abspath is provided, all paths are resolved to absolute paths based on the given path.  This allows Vulcanize to import files with local absolute URLs and allows the vulcanized file to be served from any path.
This conflicts with #70. I'll just replace this with `CHEERIO_OPTIONS` since they should be equivalent.